When a machine travels inside a tube, bearing against the walls of the tube by means, for example, of rollers, its motion is affected by deformations in the tube. In order to avoid successive impacts at abrupt variations in the surface of the walls, radial movements of the machine are damped by suitable means, for example, a spring. However under the effect of the weight of the machine, the springs lying underneath it are compressed, so that if springs are selected so that the axis of the machine coincides with that of the tube in a certain attitude of the machine, rotation of the machine about its axis will bring about a variation in the forces applied and hence a displacement of the axis of the machine with respect to the axis of the tube.
A displacement of this kind may be troublesome, particularly because of the risk of friction between one portion of the machine and the wall of the tube or even a risk of impact in certain cases, because of oscillations produced by differences in level, which could cause the undesired release of a control device on the machine. The movable machine may for example, bear a gauging device having the function of triggering plugging of the tube in which it is moving and consequently of triggering locking the machine onto the wall of the tube, upon the detection of a contraction of a certain value in the diameter of the tube.